The identification of sub-surface contamination and the determination of the area and depth of such contamination is a significant and increasing problem. Both private industry and governmental or quasi-governmental agencies recognize the growing severity of sub-surface contamination, and the attendant financial costs to clean-up such sites in order to reasonably protect the public. Real property purchasers frequently desire assurances that considered sites are not contaminated or, if contaminated, that the contamination is not migrating to new areas. While both private and public funds are being allocated to clean-up contaminated sites, there is increasing evidence that the contamination may not be "contained" as much as once believed, and that improved systems to monitor migration of sub-surface contamination may be required to properly evaluate the desired corrective action and reasonably predict the environmental clean-up costs.
Sub-surface contamination testing techniques typically involve the taking of numerous sub-surface fluid samples from various test wells, followed by the laboratory analysis of such samples. While these testing techniques are reasonably reliable, they are extremely expensive and time consuming, since a test hole must be drilled to the desired depth at each test site. To properly evaluate a property for environmental contamination and migration, hundreds of test wells might have to be drilled, and thousands of test samples might have to be analyzed. These techniques are not only expensive, but often delay by months or years the most practical utilization of the property. Surface vapor measurements are sometimes taken, since this technique is comparatively inexpensive and may be quickly performed. Those skilled in subsurface contamination recognize, however, that this technique is highly unreliable, has little utility for many contaminants, and at best is used for recently contaminated sites wherein the contamination is less than 1 or 2 meters deep.
Magnetic survey techniques have been used in recent years in an attempt to better evaluate the likelihood of recoverable hydrocarbon reservoirs by determining the depth and pattern of sedimentary rock formations containing magnetic minerals, such as magnetite. Utilizing airplanes, large area magnetic surveys have been conducted to estimate, for example, the depth to igneous rock and the thickness of sedimentary rock formations. Such magnetic survey techniques might also be used to identify structures such as faults, folds and other shifts in rock formations. This information may then be used by geologists to estimate the likelihood of recovering hydrocarbons, thereby justifying an exploratory well.
Magnetic survey techniques have also been utilized by archaeologists to assist in locating sub-surface structures or objects at relatively shallow depths of, e.g., 3 to 10 meters. In this case, researchers would monitor magnetic properties while walking over a potential site of interest. The buried materials of interest, such as a building wall or storage chest, will likely have different magnetic properties than the surrounding fill soil, which may be more or less magnetic than the structure of interest. Archaeologists are primarily interested in significant localized magnetic changes and/or magnetic readings which correspond to a presumably man-made geometric pattern. Using these techniques, it is thus possible to predict the approximate location of a structure of interest and thus reduce excavation costs.
A recent article by Machel and Burton entitled "Causes and Spatial Distribution of Anomalous Magnetization in Hydrocarbon Seepage Environments" discusses hydrocarbon seepage and migration resulting in anomalous magnetization prior to hydrocarbon invasion. The article discusses various theories, including geochemical processes and microbial processes involving aerobic or anaerobic conditions, in an attempt to explain how subsurface magnetic properties might be affected by hydrocarbons. While the primary purpose of the article appears to be the consideration that magnetic contrasts might be used in the future for hydrocarbon exploration, the paper does indicate that such magnetic anomalies may also be used to detect hydrocarbon leakage from storage tanks or waste disposal sites. Based on the information available to the inventors, no commercial activity involving this suggestion has yet occurred. The factors affecting magnetic changes when sub-surface rock is subjected to hydrocarbons are not fully understood, and techniques suggesting that magnetic surveys could be used to detect hydrocarbon reserves (as compared to rock formation depths) have not been commercially unproven.
People knowledgeable in contamination testing have long assumed that magnetic techniques could not be used to detect or evaluate sub-surface contamination, and have considered magnetic survey techniques, while perhaps hypothetically interesting, lacking practical utility. The amount of contaminate material, such as hydrocarbons, necessary to create a significant environmental problem is, of course, markedly less than the amount of material geologists and geophysists are seeking when utilizing magnetic survey technologies to locate potential mineral sites or to locate rock formations which are likely candidates for hydrocarbon exploration. Practical application of magnetic survey techniques for detecting contamination has thus not occurred. Various techniques have been proposed for reducing the cost of detecting subsurface contamination, but drilling of conventional boreholes and surface vapor measurements remain the only techniques which are widely recognized as having practical reliability.
Improved techniques are required to evaluate sub-surface contamination, and to reduce both the cost and time involved to make such evaluations. Such improved techniques have long been needed in the industry involved in evaluating environmental contamination to reduce risks, minimize clean-up costs, and provide higher assurances of public safety.